Fired at the muzzle, a representative 6.5 Creedmoor load (9.1 g, BTHP, 800 m/s) reaches roughly 20 mm (0.8 inches) into steel a36. That is enough to defeat a typical barrier of this material — treat anything behind it as exposed.
About Steel A36
Mild structural steel plate (~6 mm); car bodies are far thinner (~1 mm).